1. Technical Field
The present invention relates to a non-directional touch signal probe to be used to measure the shape of an object with a coordinate measuring machine.
2. Background Art
A coordinate measuring machine is known as a measuring instrument to measure shape, size, or the like of an object. In order to perform coordinate detection or position detection, the measuring instrument is provided with a touch signal probe wherein a touching part is provided at a top end portion of a stylus. The touching part is used to detect touching of the object.
For a touch signal probe, in a conventional example, a stylus is provided with a detecting element, such as a piezoelectric element. When a touching part at a top end portion of the stylus touches an object to be measured, impact is detected and the touch detection is performed. This conventional example is advantageous in that the configuration is relatively simple, but disadvantageous in that the sensitivity is different depending on the direction from which the touching part approaches the object. That is, it has the direction dependent property.
Conventional examples of such touch signal probes are disclosed in Japanese Patent Publication No. 60-48681 (corresponding to U.S. Pat. No. 4,177,568) (conventional example 1); Japanese Patent Laid-Open No. 54-78164 (corresponding to GB 2006435B) (conventional example 2); and Japanese Patent Laid-Open No. 8-327308 (conventional example 3).
In the conventional example 1, a measuring head or a measuring probe comprises a movable part and a stationary part, and these parts are connected to each other through a seat mechanism.
The movable part is constituted by two members divided in two. A detecting element (piezoelectric element) responding to tension and compression at high sensitivity is provided between these members.
In the conventional example 2, (a) a piezoelectric element is assembled in a part of a stylus, or (b) a stylus is divided in two and a piezoelectric element is grasped between the divided parts of the stylus, or (c) a piezoelectric element is assembled (or installed) to a touching ball provided at a top end portion of a stylus and the piezoelectric element is grasped in a connection part between a touching ball and a stylus.
In the conventional example 3, a stylus is mounted at the center of a disk-shaped substrate, and a plurality of piezoelectric elements are arranged in radial position around the stylus. In order to detect that the stylus touches the object, the sum of absolute values of signals outputted from respective piezoelectric elements is detected.
In the conventional examples 1 and 2, various cases are disclosed such that one piezoelectric element is installed or a plurality of piezoelectric elements are installed.
In general, when few piezoelectric elements are used, since the structure of a stylus is simple and the assembly is easy, an advantage exists in that the cost can be made low. But the detection accuracy is not sufficient and the direction dependent property exists.
On the contrary, when a plurality of piezoelectric elements (detecting elements) are used, although the direction dependent property becomes small by reason of combining these elements, a disadvantage exists in that the structure is complicated.
As used herein, the "direction dependent property" means the degree of difference in the response of a detecting element depending on the location of the touching portion of a touching ball at a top end portion of a stylus when the touching ball touches the object. In the conventional example 1, although the detecting elements are separated for detecting the X-axis and for detecting the Y-axis, the direction dependent property is not further considered. Consequently, improvement of the direction dependent property is not sufficient. Also, in the conventional example 2, the relation between the direction dependent property and the arrangement of the piezoelectric elements is not considered.
In the conventional example 3, the sum of absolute values of signals outputted from a plurality of piezoelectric elements is combined and a detection signal is fetched so that the directional property in the detection sensitivity is not produced when the top end portion of the stylus touches the object. However, when it touches the object from a direction orthogonal to the stylus axis, the improvement of the direction dependent property of the detection sensitivity is not sufficient. Also, the conventional example 3 does not address the case of touching from the stylus axis direction.
When coordinate detection or position detection is performed using a touch signal probe, exchanging a stylus is usually performed. In a conventional touch signal probe, however, the case of the stylus being fixed is assumed, but the case of the stylus being exchanged is not assumed.